1. Technical Field
The present invention relates generally to damping materials and particularly concerns surface coatings that enhance the reduction of vibration and noise within existing structures when applied to the surface of such structures.
2. Discussion
The presence of unwanted vibration is a common problem in the design of structures which are subject to dynamic loads. This typically occurs in military and commercial transportation systems such as automobiles, trucks, aircraft, ships, spacecraft and other vehicles. The source of vibration may be acoustic or may emanate from rotating machinery or other mechanical devices. As the manufacturing materials for these systems have become lighter and stiffer, they have tended to become more susceptible and less able to suppress vibration and noise, and as such, the need for better dynamic stability and noise suppression has become apparent. Some development effort has been directed toward measuring and improving the intrinsic damping properties of state of the art composite materials. Significant improvements have not been achieved, however, because unwarranted compromise in the static elastic properties of the materials used would result.
Active and passive damping techniques have become the subject of recent experimentation to enhance performance in the areas of dynamic load reduction, vibration and noise reduction in composite structures. Fiber reinforced composites have been attractive lightweight substitutes for metals when high specific stiffness, strength and controlled expansion are required. These composites have been particularly effective for applications in commercial and military transportation systems. Although the tailorability of composites provides an effective means for design optimization of components which must be lightweight and must meet stressing thermal and mechanical systems requirements, these structures nevertheless experience undesirable levels of vibration and noise.
Vibration frequencies which typically occur in transportation applications are in the low frequency range, typically around 1/2-200 cycles per second. Noise frequencies typically occur throughout the range of human hearing capabilities, usually stated as from about 20 Hz to about 20,000 Hz.
Current methods for reducing vibration and noise throughout these systems involve the application of a viscoelastic material (VEM) to the external surface of the structure in the form of ancillary constraining layers of coatings or tapes, much like common adhesive tape in appearance. These constraining layers dampen vibration and noise by shifting the phase of loading from that of the underlying structure, through a combination of bulk tension/compression and localized shear deformation within the applied material at the underlying structure interface. The constraining layers thus provide a secondary dynamically responsive load path such that the static properties of the primary load carrying member are not appreciably altered. In the use of such ancillary constraining layers, the properties of the viscoelastic materials are time dependent, such that they will not support any significant static load, but will react to oppose dynamic disturbances occurring within the structure to which the damping material is attached. Under quasi-static conditions the VEM will relax or creep to accommodate displacements in the load carrying member. During transient dynamic conditions the constraining VEM layer will suppress vibratory oscillations, depending on the geometry of the structure and the compliance of the VEM.
One type of current material applied to external surfaces of structures as a constraining layer for passive damping is a compliant VEM film adhered to a layer of stiff metal or composite foil, such as aluminum foil. This arrangement utilizes the compliancy of the VEM film to shift the phase of loading within the foil with respect to the loading in the primary structure.
The use of commercially available VEM materials and application techniques has resulted in a modest damping of vibration and noise, of a value generally less than 5%. One disadvantage of the use of VEM constraining layers as currently used is in the considerable weight added to the underlying structure, perhaps as much as 20-30%. Therefore, standard VEM constraining layer approaches do not offer great performance or weight efficiency for aircraft or spacecraft systems. This inefficiency arises typically from the manner in which the VEM is loaded, mainly in bulk tension/compression along with minimal localized shear at the constraining layer interfaces. In addition, the typical constraint material, such as aluminum, does not have the high stiffness to weight ratios available from fibers such as graphite.
The need therefore exists for an improved method for achieving high damping performance through the use of ancillary constraining layers of less material and weight.